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Abstract

In this paper, we discuss the realization of wavelength-division multiplexing (WDM)
transmission at high spectral efficiency. For this experiment, coherent polarization-division
multiplexing--orthogonal frequency-division multiplexing (PDM-OFDM) is used as a modulation
format. PDM-OFDM uses training symbols for channel estimation. This makes OFDM easily scalable
to higher level modulation formats as channel estimation is realized with training symbols
that are independent of the constellation size. Furthermore, because of its well-defined
spectrum OFDM requires only a small guard band between WDM channels. The dependence of the
number of OFDM subcarriers is investigated with respect to the interchannel linear crosstalk.
At a constant data rate the number of OFDM subcarriers is estimated to achieve lower linear
crosstalk in order to achieve higher spectral efficiency. We then experimentally demonstrate
dense WDM (DWDM) transmission with 7.0-b/s/Hz net spectral efficiency using 8$\,\times\,$65.1-Gb/s coherent PDM-OFDM signals with 8-GHz WDM channel spacing utilizing
32-quadrature-amplitude-modulation subcarrier modulation. Successful transmission is achieved
over 240 km standard single-mode fiber (SSMF) spans with hybrid erbium-doped fiber
amplifiers/Raman amplification.

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